This invention relates to a low voltage power supply interface apparatus for powering communication and like low voltage facilities at a power station and having wire lines connected to a remote source and more particularly a telephonic voice and data distribution system.
In various communication and like systems, analog and digital signals are transmitted over relatively low voltage lines, such as telephone lines, for communication between widely spaced and varied customers. The communication lines are often located in proximity to power distribution systems, and power stations include communication facilities with the connecting lines subject to the electrical environment at the high voltage stations. The power station ground voltage plane for the power distribution systems has an effective radius which diminishes in level with distance from the ground plane plate. Any ground fault condition can couple high voltages into the incoming communication lines. This can occur directly as a result of an integral fault in the power distribution system as well as through other sources such as lighting surges and the like. The hazardous voltage, generally referenced as a GPR voltage or ground potential rise, exists when a power line or electrical power apparatus at the power station faults to ground. The fault current flows back to electric power substation neutral or ground connections via available paths such as aerial ground wires, multi-grounded neutrals of distribution lines and the earth itself. The current in the earth returns to the electric apparatus neutrals or grounds via the power station ground grid. Since the ground grid has finite impedance, the returning fault current multipled by the ground grid impedance produces a ground potential rise or GPR at the power station ground with respect to any remote ground. For the usual telephone communication, remote ground typically would be an operating telephone company central office to which the communication lines are connected. Because of possibly large fault currents, the GPR may be thousands of volts rise at the fundamental power frequency. Such voltages present a safety hazard and risk of serious communications line equipment damage. The main station power source including the usual 120 volt A.C. supply and the station battery supply are at the station ground plane potential. Unless the station power source is connected to a remote ground connection, such as the incoming telephone wire lines from a remote telephone office, the source is inherently isolated and protected from ground potential faults. The station power source can therefore be connected to operate the station side equipment if such equipment is isolated from the incoming telephone wire lines. The communication users equipment and/or facilities are generally direct coupled to the telephone line system and, in the absence of special protective interface systems, significant damage can occur as a result of the abnormal voltage conditions in the telephone lines. The problem becomes more acute with the development and wide usage of integrated circuit electronic equipment connected to telephone lines.
Lines for communication facilities at electric power stations were historically protected by means of neutralizing transformers and isolation transformers which limit or isolate voltages of GPR between power station ground and the remote ground for the communication lines.
Neutralizing transformers provide a complete metallic wireline circuit path between station and remote ground, and the transformers buck, limit or neutralize the voltage developed by GPR between each conductor of a circuit and ground. The metallic circuit permits both A.C. and D.C. signals to be sent and received.
Isolation transformers block the conductor-to-ground GPR and pass A.C. signal voltages and current; however isolation transformers which are connected from conductor-to-conductor of a communication wire pair isolate a D.C. current path. Alternative circuitry must then be provided in those systems which require D.C. currents to pass the isolation transformer.
A more recent technique used to isolate the electric power station GPR voltage is through the use of fiber optics between the power station end of the wireline facilities and the remote end of the wireline. An electrical signal is received at the power station over wire lines from the telephone central office and converted from electrical energy to light transmission sent over varying lengths of fiber optics and thence converted back to electrical energy to drive a transducer such as a telephone set. An electrical signal is transmitted to the central office in a like manner for two way communication. One operable system is to provide the optical isolation fiber(s) and the electrical-signal to light-signal to electrical-signal all on one printed circuit card interface. The optic fiber can be chosen of sufficient length to withstand the highest expected insulation stressing voltage that may occur between the power station and remote optical interfaces.
The electrical-to-optical and optical-to-electrical interfaces require power to be suppled to them to drive the communications signal. Standard telephone set voice service is provided with remote powering over the telephone communication wireline from the battery voltage at the remote serving central telephone office. The optical/electrical interface at the power station on the station side of the optic fiber can be energized from or derived from the power station battery; thus the optical interface is energized from both sides and both battery supplies are effectively isolated by the optical fiber to protect the power station facilities. This is necessary because each battery supply relates to its own grounding means via A.C. power supply system chargers.
Various wireline circuit functions, such as data communication lines, are not supplied with battery power from the telephone central office, and are generally known as "dry circuits" as opposed to the above remote powered systems which are identified as "wet circuits" in which a D.C. current flows in the wireline as well as the A.C. signal for voice or some data signals. When "dry circuit" wirelines serve an electrial power station and an optical fiber isolation means is used to block GPR voltages, an isolated power supply is required to power the optical-electrical interface at the station on the telephone system wireless side of the optical/electrical telephone.
In summary, one prior art discloses fiber link construction for only telephone ringing and supervision signalling function. Isolation transformer interface is used for voice and data signals and all remote side isolated power as required is derived from central office battery. Another prior art utilizing all-fiber interface, which the inventor understands has been suggested, obtains isolated remote side powering by means of solar cells driven from station power light source. The former transformer art is dependent on isolation transformers with high withstand voltage. The latter solar cell art is dependent on solar cell efficiency.
There is a very significant need and demand for some form of interface or protective system for the telephone system wire line provider and the end user power station.